Upgrade Operator version in Executor deps

[RafalSkolasinski]

Sub Issue of #2088.

Current goal

Current goal is to update Operator version in Executor deps only.

Original comment below

As Executor reads SeldonDeployment CRD (provided currently via environmental variable) to be aware about the inference graph it needs to know about Operator's types like PredictorSpec and PredictiveUnit.

Currently Operator as a whole is a dependency of Executor -> which contributes a significant number of entries to go.sum.

Including only required types as Operator's subpackage (or just copying over and making sure these are in sync) will help to reduce number of dependencies in Executor significantly.

[RafalSkolasinski]

BTW, this 2509235 commit in the cruft branch shows that just copying over Operator's types removes 273 lines of deps from go.sum.

[RafalSkolasinski]

It is currently being discussed if we want to fully remove the dependency (and how could that be achieved), move to depend only on subpackage of operator (seems reasonable if possible), or just continue depending on operator.

We should probably investigating pinning operator dependency more properly than v0.0.0-20200401123312-d4c435ea5217.

[RafalSkolasinski]

Current angle is to just upgrade version of operator used by executor once operator's update is complete.